btrfs: Remove redundant assignment of tgt_device->commit_total_bytes

btrfs: Remove redundant assignment of tgt_device->commit_total_bytes

This is already done in btrfs_init_dev_replace_tgtdev which is the first
phase of device replace, called before doing scrub. During that time
exclusive lock is held. Additionally btrfs_fs_device::commit_total_bytes
is always set based on the size of the underlying block device which
shouldn't change once set. This makes the 2nd assignment of the variable
in the finishing phase redundant.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>